Who actually paid for the return ? did they send a return label or did you post it back, was all the customs paperwork done ? did it have your return address
I'd check that it wasn't stopped by customs, it might be sat at Belfast, if it looked like a "gun" on x-ray.
Presumably you included a letter with the return so if it does make it back to the seller they know what it is, what's wrong & who to refund or send a replacement to